With reduction in size and weight of electronic materials, and progress of the development of HEV and EV, requests for batteries having large capacity, high-speed charge and discharge characteristics, good cycle characteristics, and excellent safety are increasing. Among them, lithium ion secondary batteries (lithium secondary batteries) have attracted attention as the most promising battery.
However, as the premises for developing a lithium secondary battery exhibiting excellent performances, negative electrode materials, positive electrode materials, electrolytes, separators, current collectors and the like having high performances must be developed, and a battery sufficiently utilizing their characteristics must be designed.
In particular, negative electrode materials determine the basic battery characteristics, thus materials with improved characteristics such as charge-discharge capacity have been actively developed. More specifically, while the conventional negative electrode material mainly used a carbon material such as graphite powder whose theoretical electric capacity of graphite is 372 mAh/g, a combined use with other materials with theoretical electric capacities higher than graphite (for example, silicon, tin, aluminum, and the like) has been attempted, in order to obtain the higher charge-discharge capacity.
For example, Patent Literature 1 discloses a composite material of graphite and Si obtained by mixing flake graphite and silicon powder and pulverizing and granulating the mixture in a high-speed air stream. In Patent Literature 1, it is described that the specific surface area of the graphite material is preferably 0.5 to 20 m2/g or so, and graphite having a specific surface area within the above range is used in the section of Examples. In addition, Patent Literature 2 discloses a material obtained by mixing exfoliated graphite flakes and a metal such as tin.